Optically encoded phase matched second harmonic generation device and self frequency doubling laser material using semiconductor microcrystallite doped glasses

ABSTRACT

There is described a semiconductor microcrystallite doped glass that exhibits SHG, and a method of preparing, or encoding, a semiconductor microcrystallite doped glass by the simultaneous injection of fundamental and second harmonic fields, such as 1.06 μm and 532 nm. More specifically, the disclosure pertains to a structure that exhibits SHG, the structure being comprised of, by example, borosilicate glass that contains CdS x  Se 1-x  microcrystallites. Also disclosed are embodiments of devices having an optical waveguide structure formed within a glass substrate that contains semiconductor microcrystallites. The optical waveguide structure guides and contains injected radiation and also converts a portion thereof to the second harmonic. Also disclosed are optoelectronic devices that include frequency doublers, self-doubling lasant material, bichromatic optical switches, and a volume holographic medium, all of which include a glass host having semiconductor microcrystallites embedded within.

CROSS REFERENCE TO RELATED PATENT APPLICATION

This patent application is related to U.S. patent application Ser. No.07/722,345, filed Jun. 27, 1991, entitled "Second Harmonic Generationand Self Frequency Doubling Laser Materials Comprised of BulkGermanosilicate and Aluminosilicate Glasses", by Nabil M. Lawandy, nowU.S. Pat. No. 5,157,674.

FIELD OF THE INVENTION

This invention relates generally to non-linear optical devices and, inparticular, to non-linear optical devices constructed from a glassmaterial.

BACKGROUND OF THE INVENTION

Recently there has been considerable interest in glasses doped withCdS_(x) Se_(1-x) semiconductor microcrystallites. This has been due tointerest in the fundamental physics of low dimensional systems, as wellas the technologically important areas associated with optical switchingas referred to in K. M. Leung, Phys. Rev. A 33, 2461 (1986) and A. I.Ekimov et al. Solid State Comm. 69, 565 (1989). In the case ofcommercially available colloidally colored filter glasses thecrystallite size is of the order of 5-10 nm, making the crystallitelarger than the bulk exciton radius, and thus out of the quantum dotregime. These materials have been the subject of several investigationsusing four wave mixing, interferometric methods and luminescencedetection, as mentioned in R. K. Jain et al., J. Opt. Soc. Am. 73,646(1983) and M. Tomita et al., J. Opt. Soc. Am B 6,165 (1989). From thesemeasurements χ.sup.(3) (ω, ω-, ω) values have been measured which rangefrom 10⁻¹¹ to 10⁻⁷ esu. In addition, a large spread in response times,ranging from 72 μsec to 10 psec, has been observed along with anintensity dependence. Other effects which are indirectly associated withthese observations are thermally reversible photodarkening,non-quadratic dependence of phase conjugate reflectivity on pumpintensity, Franz-Keldysh oscillations, and luminescence.

It is an object of this invention to provide second harmonic generation(SHG) in glasses doped with semiconductor microcrystallites.

It is another object of the invention to provide a method of preparing asemiconductor doped glass material so as to exhibit SHG.

It is a further object of the invention to provide SHG in a silica-basedglass that contains, by example, CdS_(x) Se_(1-x) or CuClmicrocrystallites.

It is one further object of the invention to provide optical waveguidestructures, optical switching devices, and holographic memory devicesthat are fabricated with a silica-based glass that containssemiconductor microcrystallites.

It is another object of the invention to provide a lasant material thatsimultaneously lases and frequency doubles the laser radiation.

It is one further object of the invention to provide a semiconductorlaser diode that includes a frequency doubler comprised of asemiconductor microcrystallite doped glass.

SUMMARY OF THE INVENTION

The foregoing objects are realized by a semiconductor microcrystallitedoped glass that exhibits SHG, and by a method of preparing, orencoding, a microcrystallite doped glass by the simultaneous injectionof fundamental and second harmonic fields, such as 1.06 μm and 532 nm.More specifically, the invention provides a structure that exhibits SHG,the structure being comprised of a silica-based glass that is dopedwith, by example and not by limitation, CdS_(x) Se_(1-x), CuCl, PbS,GaAs, InP, ZnSe, or ZnSeS microcrystallites.

Although these composite materials have a center of inversion on amacroscopic scale, and are therefore expected to possess no second ordersusceptibility, the inventor has determined that this symmetry can bebroken, and that phase matching can be encoded, when the material issimultaneously exposed to optical radiation having a first wavelengthand a second wavelength, the second wavelength being one half of thefirst wavelength. By example, the first wavelength is 1.06 μm and thesecond wavelength is 532 nm. The radiation may be generated by amodelocked and Q-switched laser. The SHG effect is permanent in someglass/microcrystallite systems and is a strong function of the positionof the microcrystallite energy bandgap.

The use of the invention also provides a permanent, quasi-phase matched,second harmonic signal which is approximately 10⁵ times an initialbackground value. The inventor has obtained second harmonic signalswhich are visible in room lights, corresponding to a conversionefficiency of 10⁻⁶ for modelocked, Q-switched input pulses.Experimentally obtained results are presented which explain theunderlying physical mechanisms. These include polarization dependence,OIR and second harmonic preparation intensity effects, thermal erasure,and the application of external static electric fields. These resultsare shown to indicate that a most likely mechanism is an encoding of aperiodic internal electric field that results in a phase matchedElectric Field Induced Second Harmonic generation (EFISH) process.

The invention also provides an optical waveguide structure, and a methodof fabricating same. The waveguide structure provides SHG for opticalradiation propagating therethrough. The waveguide structure isfabricated through an ion exchange process in conjunction with aphotolithographic masking process. Both planar and channel waveguidesare described.

The invention also provides a laser medium, such as a glass laser rod,or optical fiber that simultaneously lases and frequency doubles thelaser radiation.

Further embodiments of the invention provide optical switching devicesand a holographic medium that frequency double an input beam wavelength.

BRIEF DESCRIPTION OF THE DRAWING

The above set forth and other features of the invention are made moreapparent in the ensuing Detailed Description of the Invention, when readin conjunction with the attached Drawing, wherein:

FIG. 1a depicts an enlarged view of a glass host material havingsemiconductor microcrystallites contained within;

FIG. 1b shows optical apparatus for preparing a semiconductormicrocrystallite doped glass for SHG;

FIG. 2 shows a time dependence for SHG preparation in a 1 mm thick OG550 filter illuminated with 2 W and 50 mW of 1.06 um and 532 nmradiation, respectively;

FIG. 3a illustrates a dependence of SHG on input radiation intensity for1 mm thick samples;

FIG. 3b illustrates a log-log plot showing a quadratic relationship ofSHG to input intensity;

FIG. 4 shows a relationship between SHG and a length of prepared sample;

FIG. 5 displays relative conversion efficiencies of differentcolloidally doped filters;

FIG. 6 illustrates SHG as a function of input polarization relative towriting polarization for an OG 550 filter;

FIG. 7 depicts SHG dependence on 1.06 μm radiation preparationintensity;

FIG. 8 depicts SHG dependence on 532 nm radiation preparation intensity;

FIG. 9 depicts a SHG erasure effect accomplished with 1.06 um radiation;

FIG. 10 depicts a SHG erasure effect accomplished with 532 nm radiation;

FIG. 11 depicts thermal SHG erasure as a function of time for threedifferent ambient temperatures;

FIG. 12 depicts an energy band diagram that illustrates a mechanism forSHG preparation in a semiconductor microcrystallite doped glass;

FIG. 13a is a plan view showing a spiral waveguide fabricated within asurface region of a semiconductor microcrystallite doped glass;

FIG. 13b is a cross sectional view showing the waveguide of FIG. 13a;

FIG. 14 is a cross sectional view showing an optical device thatincludes a frequency doubler constructed in accordance with theinvention;

FIG. 15 is a graph illustrating a change in bandgap as a function ofcomposition of a Ge-Si alloy;

FIG. 16 shows a holographic medium that is constructed and operated inaccordance with the invention;

FIG. 17 is a block diagram showing apparatus for preparing a frequencydoubling glass laser rod;

FIG.18 shows a block diagram of a frequency doubling laser that includesa semiconductor microcrystallite glass rod prepared in accordance withFIG. 17; and

FIG. 19 shows a top view of a bichromatic logic switching device.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a illustrates a volume of silica-based glass host material 1having a plurality of microcrystallites 2 embedded therein. Themicrocrystallites are comprised of a semiconductor material. Theinvention is described below primarily in the context of a borosilicateglass host material having CdS_(x) Se.sub.(1-x) microcrystallitescontained therein. These microcrystallites are uniformly distributedthroughout the glass host material and have a nominal spacing betweenthem that is a function of the concentration of the microcrystallites.It should be realized, however, that the teaching of the invention isnot to be construed to be limited to only this material combination orto uniform distributions of microcrystallites. For example, a glass hostmay include semiconductor PbS, CuCl, GaAs, InP, ZnSe, or ZnSeSmicrocrystallites. Furthermore, the concentration of themicrocrystallites may be other than uniform for providing SHG at onlyselected portions of the glass host material.

As employed herein, a semiconductor microcrystallite or crystallite isconsidered to be a single crystal or a polycrystalline aggregate of asemiconductor material having an energy band structure. Aggregates whichexhibit bulk, as well as quantum dot behavior, are included within thisdefinition.

The CdS_(x) Se.sub.(1-x) microcrystallites 2 may be present in aconcentration of approximately 0.3 mole percent to concentrations up to50 mole percent or greater. The greater the mole percent concentrationthe greater is the SHG effect. The microcrystallites 2 are randomlyoriented and have dimensions on the order of approximately 100 angstromsto approximately 200 angstroms. Although CdS_(x) Se.sub.(1-x) isnoncentrosymmetric, the random orientations of the crystallites 2 resultin χ.sup.(2) =0 for the composite system. This result is exploited bythe invention to provide SHG in the manner described in detail below.

The glass host 1 may also contain Na or K in a concentration range ofapproximately 5 mole percent to 20 mole percent. Nd may also be presentin a concentration of, for example, 1.5 percent. The invention alsoprovides for the construction of glass laser rods or optical fibers thatprovide a fundamental frequency and twice the fundamental frequency.This aspect of the invention is described in detail below.

Each microcrystallite 2 retains, within the glass host 1, the basicproperties of the bulk semiconductor. Also, the nonlinear susceptibilityof the microcrystallites, χ.sup.(3) mc, is greatly enhanced for abovebandgap excitation. For example, quoted values of χ.sup.(3) (ω₂ -2ω₁ ;ω₁, ω₁, -ω₂) for CdS at λ=0.694 μm and λ=0.53 μm are 2.24×10-20 m² /V²and 1.05×10-17 m² /V², respectively. The second wavelength, which is atthe band edge of CdS, results in a χ.sup.(3) mc which is 10³ times theoff-resonance value, and approximately 10⁵ times larger than that ofsilica. Thus, if electron dynamics within the microcrystallites 2 areconsidered, internal optical rectification fields as large as 10⁷ V/mare expected, which includes the static dielectric constant of CdS,ε˜8.9 for above-gap excitation. Such large fields result in energyincreases as large as several tenths of an electron-volt across themicrocrystallite.

For the purpose of characterizing the SHG effect with microcrystallitedoped glass materials, experiments were performed on optical filters ofa type manufactured by Schott Glass; the filters ranging from GG 495 toRG 630 and having a variety of thicknesses (1 mm to 5 cm). These filtersare comprised of a glass host doped with CdS_(x) Se.sub.(1-x)microcrystallites. The filter nomenclature is such that the number, suchas 495, gives the approximate semiconductor bandgap in nanometers.

One possible mechanism for the SHG encoding process is shown in FIG. 12.The diagram represents a basic energy level structure for a direct gapsemiconductor and its relation to the surrounding glass. The primaryoptical encoding steps are believed to be: 1) optical excitation of theelectron to the conduction band (E_(c)); 2) motion of the electron underthe influence of the internal optical rectification field, establishinga wavefunction pinned against one side of the crystallite; and 3)trapping in an "exterior" deep trap of energy Et. Measurements onphoto-ionization of CdS microcrystallites in glasses have demonstratedsuch trapping when above bandgap excitation was employed, and indicatethat the trap site is most likely a deep electron trap in the glassmatrix near the crystallite surface. The optical encoding, describedbelow, is believed to add directionality to this basic process.

In FIG. 12 the arrow designated 4 shows a thermal SHG erasure viaionization to the conduction band, and arrow 5 indicates an optical SHGerasure mechanism via direct absorption. The arrow 6 indicatesluminescence from interior surface trapping sites. The thermal andoptical SHG erasure mechanisms are described in further detail below.

Optical apparatus for preparing semiconductor microcrystallite dopedglasses (SMDG) for SHG is shown FIG. 1b, wherein P1 and P2 arepolarizers; L1 and L2 are 10 cm lenses; S is a microcrystallite dopedglass sample; BFP is a 532 nm bandpass interference filter; and PMT is aphotomultiplier tube.

More specifically, the apparatus includes a modelocked, Q-switched andfrequency doubled Nd:YAG laser 10, a KTP crystal 11, a 10 cm focussinglens 12, cross-polarizers 14 and 16, and a phase sensitive detectionsystem 18 capable of detecting 10⁻¹⁴ W of average power. The laser 10produces pulses that are 120 psec and 90 psec in duration at 1.06 μm and532 nm, respectively, with a 76 MHz modelocking rate and Q-switched rateof 1 kHz. The pulses incident on a SMDG sample 20 are linearly polarizedand are focussed to a measured spot size 30 μm in diameter (for 1.06 μmradiation). The laser/cross-polarizer system delivers up to 3 Watts ofaverage power at 1.06 μm, and up to 1 watt at 532 nm. The two beams arenot separated in order to minimize any relative phase jitter effects dueto dispersive thermal index effects in beam separation and recombinationoptics.

The second harmonic signals are detected using a lens 22 and up to fourband pass filters (BPFs) 24. A photomultiplier tube (PMT) 26 gain isheld constant throughout all experiments. The signals from preparedsamples were measured using calibrated neutral density filters. Inaccordance with the invention typical values of background SHGcorresponded to a conversion efficiency of approximately 10⁻¹³ to 10⁻¹²,with 1 W of incident average power at 1.06 μm.

It is noted that the apparatus described above provides preparation forCdS_(x) Se_(1-x) and employs 1.06 μm and 532 nm radiation. However, forother semiconductor microcrystallites other wavelengths are appropriate.For example, for CuCl wavelengths of approximately 0.7 nm andapproximately 0.35 nm are employed. In general, the fundamentalwavelength is within a range of approximately two micrometers toapproximately 0.5 micrometers, and the second harmonic wavelength is onehalf of the fundamental.

Initial experiments were performed on one mm thick OG 550 filter glassin order to determine a time evolution of the SHG process. Thepreparation process was interrupted periodically to read out the secondharmonic power. FIG. 2 shows the time evolution for the OG 550 filterexposed to 2 W and 50 mW of average power at 1.06 μm and 532 nm,respectively. The results show that the SHG increases by 10⁵ and thatsaturation occurs on the time scale of a few hours. Similar experimentswere performed on a non-resonant bandgap filter (GG 495) and resulted inthe same basic time evolution.

In order to further verify that a second order process was indeedresponsible for the signals, a dependence of the SHG signal on inputpower was determined. The results are shown in FIG. 3a for the OG 550and a GG 495 filter. The GG 495 filter was found to have negligibleabsorption at 532 nm (α=0.03 cm⁻¹). This material is expected to be lesssensitive to pumping and readout induced index changes from carrierexcitation and thermally effected shrinkage of the energy gap, as notedin J. I. Pankove, Optical Processes in Semiconductors, DoverPublications, Inc., New York (1971), p. 27. The dependence of SHG on IRpower for the GG 495 filter was determined from a least squares fit to alog-log plot of the data in FIG. 3a. FIG. 3b shows the transformed dataand indicates that the process is dependent on I(ω)¹.98 with acorrelation greater than 0.99.

Based on these results it is clear that an effective second ordersusceptibility (χ.sup.(2)) is induced in these materials. The deviationsfrom second order behavior in the OG 550 filter are believed to be dueto direct and indirect intensity dependent phase matching effects. Inaddition to the semiconductor doped glasses, a single crystal sample ofCdS was also examined. This sample exhibited a preparation inducedincrease in SHG of a factor of two. The result serves to demonstratethat the observed increase of several orders of magnitude observed inthe SMDG is unique to the microcrystallite guest-glass host system.

In order to further verify that a phase matched process occurs in theSMDG the scattered sidelight at 532 nm from the GG 495 filter, preparedalong five centimeters, was examined using the optical system shown inFIG. 1b. FIG. 4 shows the growth of the second harmonic beam along thepropagation axis of the SMDG. The filter was GG 495 prepared for fourhours with 1 W and 1.5 mW of 1.06 μm and 532 nm radiation respectively.Maximum conversion efficiency corresponds to 5×10⁻⁷. Although thedependence on length is not perfectly quadratic, the result serves toillustrate that phase matching occurs. The second harmonic conversionefficiency of this SMDG material, after nine hours of preparation, wasfound to be 5×10⁻⁷. This value, along with the input beam parameters,results in a χ.sup.(2) of the order of 10⁻¹⁶ m/V.

In addition to the two SMDG materials discussed thus far other SMDGmaterials were evaluated to determine the role of resonance. For GG 495through OG 590 filters the experiments were performed on one mm thickfilters with identical preparation and readout processes. The results inFIG. 5 show the SHG efficiency of nine filters, where the OG 550 filterprovides an increase of approximately four orders of magnitude. In FIG.5 the squares are measured values, and the solid line is a best fitbased on the model presented below. The GG 400 (plotted as a point at400 nm), GG 450 and GG 475 filters were two mm thick, and all otherfilters were one mm thick. It appears that resonance strongly enhancesthe SHG effect, as will be described.

In a further experiment the dependence of the output second harmonictransverse beam structure on the writing second harmonic transverse beamprofile was examined. By adjusting the KTP crystal 11 at a steep angle,the second harmonic generated by the 1.06 μm Gaussian beam emerged as adouble lobe pattern, due to the KTP crystal 11 birefringence. When thisbeam was used in conjunction with the uniform 1.06 μm Guassianfundamental beam to prepare the GG 495 filter, it was found that thesecond harmonic signal generated during readout was double lobed. Whenthe second harmonic was adjusted to have a uniform profile, the readoutsecond harmonic emerged in a solid mode as well. This behavior ofslaving the output SHG to the encoding beam pattern is identical to theeffect observed in germanosilicate optical fibers, except that in theSMDG material there are no modal constraints as there are with opticalfibers.

The dependence of the SHG output on input polarization, relative toencoding polarization, is shown in FIG. 6 for the three mm OG 550filter. Squares are measured values, and solid diamonds are proportionalto cos² (θ). Points are scaled to account for optical erasure whilereading and are corrected to account for a temporal decay of the SHGinherent in the readout process. The output power at 532 nm includesboth output polarizations. SHG is seen to behave as cos² (θ), where θ isthe angle between writing and reading polarizations. An expected cos⁴(θ) dependence may be masked by the summing over three tensor elements,all of which contribute to SHG for the linearly polarized inputradiation.

It is noted that no self-preparation (i.e. preparation with no secondharmonic seed radiation) was obtained over the course of a twelve hourperiod, even at intensities just below the damage threshold of the SMDGmaterial (˜500 W/μm²).

In order to better understand the process of induced SHG in the SMDGmaterial the intensity dependence of the preparation process wasexamined. Experiments were performed with GG 495 and OG 550 filters as afunction both of 1.06 μm and 532 nm incident powers. The experimentswere performed on one mm thick samples. Portions of the SMDG materialwhich had never been exposed were prepared for a measurement and eachexposure was limited to 20 minutes maximum. In order to minimize phasematching changes between preparation and readout, the readout IR powerat each point was set to the writing value.

FIG. 7 shows the results of the twenty minute preparation as a functionof the average IR power, with the second harmonic seed power heldconstant at 10 mW. The OG 550 filter was three mm thick, and points wereprepared for 20 minutes with 20 mW of 532 nm light in addition to theindicated IR power. The GG 495 filter was one mm thick, and points wereprepared for 10 minutes with 5 mW of 532 nm light. It is important tonote that the readout and writing powers are the same. Log-log plots ofthe data in the unsaturated region (<1 watt) revealed that the χ.sup.(2)χL product scales as jE(ω)¹.48 and E(ω)².15 for the GG 495 and OG 550filters, respectively. SHG values were normalized assuming square lawdependence on readout intensity.

FIG. 8 shows the results of the twenty minute preparation as a functionof the average second harmonic seed power with the fundamental averagepower held constant at one watt. The GG 495 filter was 3 mm thick, andpoints were prepared for 10 minutes with two W of 1.06 μm radiation inaddition to the indicated power at 532 nm. The OG 550 filter was one mmthick, and points were prepared for 20 minutes with 2 W of 1.06 μmradiation. The results indicate that there is a sharp rise, a maximum,and a region of decreasing SHG. This is indicative of an erasuremechanism, which is believed to be qualitatively similar to behaviorobserved in a germanosilicate fiber, as referred to by F. Oullette, K.O. Jill and D. Johnson, Optt. Let. 13,515 (1988).

As was noted, the second harmonic dependence in the preparation stageindicates evidence of an erasure mechanism. GG 495 and OG 550 filters,each three mm thick, were prepared for 20 minutes with 2 W and 50 mW offundamental and second harmonic powers, respectively. Once prepared, itwas observed that over a period of several days no apparent decay couldbe observed when the samples were maintained under ambient conditions(25° C.) in the absence of illumination. However, when the preparedsamples were read out with IR radiation only, the signal decayed withtime. FIG. 9 shows the decay of both the OG 550 and GG 495 samples. Bothfilters were three mm thick and were illuminated with two W of 1.06 μmradiation. The OG 550 exhibits a rapid decay over a period of fifteenminutes, while the OG 495 decayed by only a few percent over the sameperiod. Fitting exponential decays to the data gives a decay rate of1.5×10⁻² sec⁻¹ for the GG 495 filter at two W of average readout power.The OG 550 gives values of 4.1×10⁻² sec⁻¹ and 7.7×10⁻² sec⁻¹ at two Wand three W, respectively. From the decay rates it appears that theerasure effect has a near quadratic dependence on IR power.

In addition to the measurements using 1.06 μm radiation the erasureprocess was examined using the second harmonic (532 nm). The decay ofthe second harmonic with time for three different average powers isshown in FIG. 10. The filter was OG 515 having a thickness of three mm.Analysis of the 0.5 watt case shows that the decay cannot be describedby a single exponential. The curves exhibit a decay rate which decreaseswith time and is of the order of 10⁻³ sec⁻¹ for 0.5 W of average power(0<t<10 min.)

The induced SHG effect was found to be permanent on a time scale ofseveral days under dark conditions at room temperature. This impliesthat if trap states are responsible for the encoding process, they aredeep enough to account for the long lifetime under ambient conditions.In order to determine the activation energy involved, a thermal erasureof the OG 550 filter was examined. FIG. 11 shows the decay of the SHGsignal in the OG 550 filter as a function of time for three differenttemperatures. All samples were 1 mm OG 550 filters, and were read withtwo W at 1.06 um. The decays are well approximated by exponentials andresult in an activation energy of 0.6 eV.

The large effect that heating has on the signal decay complicates theinterpretation of the optical erasure results, since the locallyirradiated region experiences a temperature increase with both IR andgreen (532 nm) illumination. Measurements were made of the localtemperature increase for the two filters. When one W of IR was incidenton the one mm filters, the GG 495 temperature 1 mm from the beam centerincreased by 3.1 K., and the OG 550 by 3.0 K.

Illumination with 0.1 W of 532 nm light resulted in temperatureincreases of 0.3 and 0.5 K. for the GG 495 and OG 550 filters,respectively. Thus, heating plays a roll in optical erasure, but is mostlikely not the dominant mechanism.

The results discussed thus far strongly favor the encoding of a periodicsymmetry breaking phenomenon, most probably an internal electric field.This encoding is believed to find its origin in a nonlinear holographicprocess, where the spatial phase information is carried by thefundamental and second harmonic waves. Models for similar behavior ingermanosilicate fibers suggest that optical rectification fields of theform:

    χ.sup.(3) (0;ω,ω,-2ω)E.sup.2 (ω)E*(2ω)e.sup.i Δkz,

where Δk=2k(ω)-k(2ω), in the bulk material is responsible for theencoding, as published by R. H. Stolen and H. W. K. Tom, Opt. Lett. 12,585 (1987). The χ.sup.(3) in silica is very small (10-² -10-²² m² /V²)and results in approximately one V/cm fields in the fiber. In theprepared SMDG filters of the invention, however, the composite χ.sup.(3)(0;ω,ω,-2ω) is believed to be much larger, especially near themicrocrystallite band edge.

In order to place some lower estimate on the internal field, the effectof an applied external field was also investigated. Experiments wereperformed on OG 550 and GG 495 filters between transverse electrodes.Both samples were prepared with the optical field polarization parallelto the applied electric field, and the GG 495 was also prepared withlight polarized perpendicular to the applied field. The application offields as large as 10⁶ V/m during preparation and readout resulted in nomeasurable change in SHG conversion efficiency. It is thereforeconcluded that if the encoding process is viewed in terms of aneffective optically generated d.c. field, then this field is largecompared to 10⁶ V/m.

The results presented above on the length and readout intensitydependence give evidence that a second order phase matched nonlinearinteraction takes place in the prepared samples. The results of theindex-summed nonlinear susceptibility tensor properties, determined byvarying the readout polarization, are consistent with the presence of asymmetry breaking electric field within the material.

A most likely process for increased interaction length is quasi-phasematching. This mechanism requires a periodic effective nonlinearsusceptibility given by:

    χ.sup.(2) =χo.sup.(2) cos (Δkz+φ),       (1)

where Δk=2k(ω)-(2ω), and φ is a constant phase. Combining this phasematching process with the presence of an internal electric field, Edc(z)to break symmetry, leads to

    χ.sup.(2) ˜χ.sup.(3) (-2ω;ω,ω,0)E.sub.dc cos (Δkz+φ),                                    (2)

where χ (3 is the third order susceptibility tensor for the compositecrystallite glass material, and E_(dc) is the amplitude of the internalfield encoded by the writing beams. The polarization experiments requirethat E_(dc) point along the direction established by the polarization ofthe writing beams.

The preceding discussion shows that symmetry can be broken andquasi-phase matching can occur if the optical encoding process resultsin the establishment of a permanent periodic electric field. The resultson the response of glasses doped with varying relative concentrations ofS and Se, to tune the crystallite bandgap, reveal a preparationresonance of approximately 550 nm. The bandgap can also be excitonicallytuned using quantum size effects when the particles are smaller than theexciton radius. This may be controlled by the glass striking conditions.The increase of SHG as the bandgap moves closer into resonance from thelong wavelength side indicates that carrier excitation is required. Thedecrease in SHG after the resonance may be a consequence of absorptionin the writing and readout process. By example, FIG. 15 shows the effecton bandgap of a Ge-Si alloy for changes in composition of the Ge and Sicomponents.

The invention has been presented in terms of the results of a variety ofmeasurements on optically encoded second harmonic generation in CdS_(x)Se_(1-x) doped glasses. The results indicate that effective χ(2) valuesas large as 10⁻¹⁶ m/V are attainable from commercially available filterglasses. This value, along with 5 cm of active length, results in a 10⁶conversion efficiency, and the generation of a second harmonic signalthat is visible in room lights. The effectiveness of one specific typeof glass filter over another is believed to be due to Fe impuritieswhich provide more election trapping sites to lock-in the field.

It should be realized that the use of the invention is not restricted toonly the commercially available borosilicate filter glasses describedthus far. That is, for a given application the glass host and theparticular semiconductor microcrystallite and the concentration thereofmay be explicitly defined and fabricated. Also, the use of the inventionis not restricted to the bulk, monolithic forms of the glass hostmaterial as is typically provided in a filter glass material. That is,the glass host material containing semiconductor microcrystallites maybe provided as a coating or layer upon a substrate. By example, asemiconductor doped glass is sputtered into a thin film with dopingdensities of, for example, 30 percent. Such a film or layer may beintegrated with, by example, a conventional laser diode so as tofrequency convert the output thereof, after suitable preparation.

By example, and referring to FIG. 14, there is shown an optical device20 that includes a substrate 22 and a frequency doubler 24. Thefrequency doubler 24 is comprised of a glass containing semiconductormicrocrystallites of the type described above. Device 20 includes, byexample, a semiconductor diode laser 26 positioned for radiating thefrequency doubler 24. Laser 26 may be of conventional constructionhaving an active region 28 that is bounded by cladding layers 30a and30b. A pair of electrodes 32a and 32b are provided for coupling thelaser diode 26 to a source of power, schematically shown as a battery34. The semiconductor laser diode 26 has an output wavelength of 850 nm.In accordance with the invention the frequency doubler 24 is prepared asdescribed above so as to generate 425 nm radiation from the input 850nm. Suitable semiconductor microcrystallite compositions for doubling850 nm include CdS or CuCl, and related alloys including a third elementsuch as in CdSe_(x) S_(1-x) semiconductors.

Preferably, the frequency doubler 24 is deposited as a film or coatingupon the substrate 22 by sputtering or an equivalent technique. However,the frequency doubler 24 may be bonded to the substrate by an epoxy orany suitable adhesive. In like manner, the substrate 22 may be asubstrate that the laser diode 26 is fabricated upon, or the laser diode26 may be attached to the substrate by an epoxy or any suitableadhesive. The total length L of the frequency doubler 24 need not be anylonger than an amount of the bulk glass that is prepared for SHG by theabove described method. For example, L may be equal to approximately 0.5mm. The frequency doubler 24 may be prepared, after deposition, byirradiating the face of the frequency doubler 24 that is opposite theoutput face of the laser. The irradiation of the frequency doubler 24can be accomplished with a system as seen in FIG. 1b modified to includea Ti-Sapphire laser tuned to 850 nm and a frequency doubler forproviding the 425 nm radiation. If the frequency doubler 24 is otherwiseattached to the substrate 22, the frequency doubler 24 may be preparedas in FIG. 1b, as modified above, and then subsequently bonded to thesubstrate 22.

The optical device 20 thus includes a SHG wavelength converter of smallsize and high efficiency for converting the near IR output of the diode26 to blue green light. One application for such a device is in opticaldata storage readout systems wherein it is desirable to minimize theoptical wavelength so as to increase the bit packing density of themedia.

Further in accordance with the invention, and referring to FIGS. 13a and13b, there is described an optical waveguide that provides SHG.

Specifically, a waveguide 36 is formed in a bulk glass substrate 38. Thewaveguide is defined by a channel region 36a having an index ofrefraction that is larger than the index of refraction of thesurrounding substrate 38. This results in a guiding and confinement ofinjected radiation about the waveguide channel. In accordance with theinvention a portion of the guided radiation is frequency doubled by theSHG effect resulting from semiconductor microcrystallites embeddedwithin the substrate 38.

Two examples of the fabrication of the waveguide 36 are now presented.

EXAMPLE 1

Low pass filters manufactured by Corning Glass, numbers 3-70 (514 nm)and 3-71 (493 nm), were placed in a KNO₃ melt at 400° C. for 20 hours.These filters had Na₂ O concentrations of 14.3 percent and 14.4 percent,respectively. Planar waveguides were fabricated through an ion-exchangeprocess to have a depth of 15 μm. Channel waveguides were fabricated byphotolithographically masking the glass surface with aluminum duringdiffusion. The mask provided open diffusion channels having a width of60 μm. From an effective mode size determined by an output diffractionpattern, in conjunction with measured index of refraction changes, thewaveguide depth was determined to be 15 μm.

EXAMPLE 2

A filter glass (Schott 495) was employed, the glass containing a smallamount of Na+ and 20 percent K+. Two samples were placed in a 350° C.melt of RbNO₃ for 22 hours and 41 hours, respectively. Planar waveguideswere made with a depth of 25 μm and 41 μm, respectively. Channelwaveguides were fabricated, with an aluminum mask, to have dimensions of65 μm by 30 μm.

It was found that ion-exchange in these systems resulted in a smallerindex of refraction difference in spite of the higher amount ofpotassium.

This may be explained in terms of a simple model which scales the indexchange by (a) an amount of ions to be exchanged, and (b) a change inpolarizability caused by replacement of ions with larger radii.

This is shown by the equation: ##EQU1## where, R_(Na) =1.57,R_(K)=2.03,R_(Rb) =2.16, N_(Na) =14.4%,N_(K) =20%.

The waveguides, prepared by the first and second examples describedabove both provided SHG, after preparation, when a 1.06 μm beam wasinjected. Preferably, a grating structure 38a is provided at a terminalend of the spiral waveguide to enable extraction of the fundamental λ₁and the second harmonic λ₂.

The use of the SHG effect, in conjunction with an ion-exchange waveguidefabrication technique in the glass host, results in an integration ofthe optical switching capabilities of these materials with efficient,low cost frequency doubling. As an example, and referring to FIG. 19,there is shown a bichromatic logic switching device 50 that includes twochannel waveguides 52 and 54, fabricated as described previously withina surface of a semiconductor microcrystallite doped glass substrate 51.The waveguide 52 is not prepared to generate the second harmonic, whilethe waveguide 54 is prepared, as described above, to generate the secondharmonic. The waveguides approach one another within a region designatedby A and are spaced apart at a distance of ##EQU2## where λ is thefundamental wavelength propagating in waveguide 52, n_(o) is the indexof refraction of the glass host, and n_(c) is the index of refraction ofthe cladding. The spacing between the waveguides is thus generally onthe order of the mode confinement length. At high intensity (I>I_(c))radiation propagating in waveguide 52 couples into the waveguide 54 in aknown fashion, where I is the intensity of the radiation propagating inwaveguide 52 and I_(c) is a critical intensity.

In accordance with the invention, when I>I_(c) a portion of the coupledradiation of the fundamental (λ₁) is converted to the second harmonic(λ₂). A filter 56 that is transmissive at λ₂ is provided at the outputof the waveguide 54. A detector 58 is positioned for detecting thepresence of the second harmonic. If the detector 58 detects the presenceof the second harmonic it is indicated that I>I_(c). As a result, it isunnecessary to spatially resolve the outputs of the two waveguides 52and 54 so long as the presence of the second harmonic is detected. Thefilter 56 may be photolithographically formed at the terminal end ofwaveguide 54, or may be provided as a separate component.

It is now shown that the provision of SHG in semiconductormicrocrystallite doped glasses, as taught by the invention, furtherenables the use of a laser rod or optical fiber to generate afundamental wavelength and to also generate a frequency doubledwavelength.

By example, a common and most useful glass laser is Nd:Glass, where Ndis doped at 1-5 wt % into a base glass with, for example, 66 wt % SiO₂,16 wt % Na₂ O, 5% BaO, 2 wt % and 1 wt % Sb₂ O₃. In this regardreference is made to E. Switzer and C. G. Young "Glass Lasers" in LasersVol. 2, A. K. Levine ed., Marcel Dekker Inc., N.Y. (1968) p. 191.

One recipe of interest herein includes Nd, or any other well knownlaser-ion such as Tm³⁺, Er³⁺, Nd³⁺, Yb³⁺, or Ho³⁺, in a silica-baseglass that includes semiconductor microcrystallites, such as CdS_(x)Se_(1-x).

Such a laser/doubler may be prepared as follows.

Referring to FIG. 17 there is illustrated a laser rod preparation system40 that includes a laser cavity 42 bounded by reflective mirrors 44a and44b. A laser rod 46 to be prepared for SHG is installed in the cavity 42and is optically coupled to a flashlamp 48. An optical frequencydoubling component, such as a KTP crystal 50, is provided within thecavity 42. Mirror 44a is 100% reflective at the fundamental wavelength(ω) and mirror 44b is 100% reflective at ω and 2ω. By example, thefundamental wavelength is 1.06 μm and the harmonic is 532 nm.

The laser rod 46 is pumped by the flashlamp 48 and operated for a periodof time of from several minutes to several hours with the KTP crystal50. This produces a large ω field and a 2ω field and prepares thelaser/doubler for SHG in a manner similar to the injected 1.06 μm and532 nm used to prepare the sample 20 of FIG. 1b.

Referring to FIG. 18, after the laser rod 46 has been prepared theoutput mirror 44b is replaced with a mirror 44c that is 100% reflectiveat ω and substantially transparent at 2ω. The KTP crystal 50 is removed,and the laser is operated to simultaneously produce 1.06 μm and 532 nm.In that the mirror 44c is substantially transparent to the secondharmonic the coherent optical output of the laser is at twice thefrequency of the laser rod fundamental frequency. In addition, theefficiency is high since the intracavity field at 1.06 μm is very large.

It is within the scope of the invention to remove the prepared rod 46from the cavity 42 and install same within another laser cavity. It isalso within the scope of the invention to provide the mirror 44c suchthat it is partially transmissive to the fundamental frequency, therebyproviding both 1.06 μm and 532 nm at the output.

Referring to FIG. 16 there is shown a further embodiment of theinvention, specifically, a transmission holographic medium 60. Medium 60is comprised of a semiconductor microcrystallite doped glass and hastypical dimensions of one centimeter on a side. The medium 60 isprepared and recorded with a preparation beam that includes λ₁ and thesecond harmonic λ₂, and is read out with a readout beam having awavelength of λ₁. The holographic medium 60 has a plurality of volumeholograms stored within that are stored by illuminating a region of themedium with λ₁, such as 1.06 μm, while λ₂ is provided to reflect off ofan object or pattern to be recorded before entering the medium 60. Themedium 60 is exposed to both wavelengths for a period of time sufficientto provide a desired degree of preparation. As a result, simultaneousrecording and preparation occurs. Subsequently, when the readout beam isapplied to a previously recorded region, an output beam, correspondingto a selected one of the volume holograms, is output at a wavelength ofλ₂. As a result, the holographic medium 60 provides a frequency doubledoutput. That is, when illuminated with, for example, 1.06 μm radiation,the holographic medium produces a green image.

The erasure mechanism described above can be beneficially employed toerase a selected one or to erase all of the volume holograms storedwithin the medium 60. As a result, the medium 60 may be written with newinformation. Optical erasure may be accomplished using anotherwavelength that is short enough to pump the bandgap of the semiconductormicrocrystallites embedded within the medium 60. For example, theerasure beam wavelength may be approximately 4000 Angstroms. The erasuremay also be accomplished, depending on the glass host/microcrystallitecomposition, with λ₁, λ₂, and/or thermally. By what ever erasuremechanism is employed, a random access read/write optical memory isprovided. By directing the erasure beam to a selected region, only theinformation stored within that region is erased. The medium 60 may alsobe partially or totally erased by blanket illuminating a selectedportion or the entire volume of the medium 60.

In conclusion, it has been shown that centrosymmetric glasses doped withCdS_(x) Se_(1-x) microcrystallites may be optically prepared to producea phase matched second harmonic generation process. This inventionextendable to the quantum dot regime, where quantum confinement resultsin larger nonlinearities, and to other semiconductors, than thosespecifically mentioned above. Other wavelengths may also be employed toprepare and readout the material. The bulk glass may be provided as amonolithic body, as a film, or as a coating applied to a substrate.Sputtering is one suitable process for fabricating the coating. In thisregard the material may be integrated with a laser diode device toprovide a fundamental and a second harmonic output.

While the invention has been particularly shown and described withrespect to a preferred embodiment thereof, it will be understood bythose skilled in the art that changes in form and details may be madetherein without departing from the scope and spirit of the invention.

What is claimed is:
 1. A method of preparing a material so as to exhibitsecond harmonic generation for optical radiation incident on thematerial, comprising the steps of:providing a glass host havingsemiconductor microcrystallites contained within; and irradiating theglass host with radiation having a first wavelength and a secondwavelength that is one-half of the first wavelength, the glass hostbeing irradiated for a period of time sufficient to obtain a desiredvalue of conversion efficiency of the first wavelength into the secondwavelength.
 2. A method as set forth in claim 1 wherein the glass hostis comprised of a silica-based glass and wherein the semiconductormicrocrystallites are comprised of CdS_(x) Se_(1-x), wherein x has avalue within a range of zero to one.
 3. A method as set forth in claim 2wherein the silica-based glass further includes a material selected fromthe group consisting of Na, K, and Nd.
 4. A method as set forth in claim1 wherein the first wavelength is within a range of approximately 2 μmto approximately 0.5 μm and wherein the second wavelength is one half ofthe first wavelength.
 5. A method as set forth in claim 1 wherein thesemiconductor microcrystallites have a density of approximately 0.3 molepercent to approximately 50 mole percent.
 6. An optical devicecomprising:a source of optical radiation having a first wavelength; andmeans, optically coupled to an output of said source, for converting theoutput of said source to a second wavelength that is one-half of thefirst wavelength, said converting means including a glass host havingsemiconductor microcrystallites embedded within, said converting meanshaving an internally generated periodic quasi-phase matched electricfield that frequency doubles the output of the source.
 7. An opticaldevice as set forth in claim 6 wherein the bulk glass is comprised of asilica-based glass and wherein the semiconductor microcrystallites arecomprised of CdS_(x) Se_(1-x), wherein x has a value within a range ofzero to one.
 8. An optical device as set forth in claim 7 wherein theboroslilicate glass further includes a material selected from the groupconsisting of Na, K, and Nd.
 9. An optical device as set forth in claim6 wherein the first wavelength is within a range of approximately 2 μmto approximately 0.5 μm and wherein the second wavelength is one half ofthe first wavelength.
 10. An optical device as set forth in claim 6wherein the microcrystallites are selected from the group consistingessentially of CdSSe, GaAs, InP, ZnSe, CuCl, PbS, and ZnSeS.
 11. Anoptical device as set forth in claim 6 wherein said source includes asemiconductor laser diode.
 12. An optical device as set forth in claim 6wherein said source includes a Nd:YAG laser.
 13. An optical device asset forth in claim 6 wherein said converting means is deposited upon asubstrate.
 14. An optical device comprising a laser diode means havingan output for providing radiation at a wavelength λ₁, said opticaldevice further including a frequency doubler means that is opticallycoupled to the output of said laser diode means, said frequency doublermeans being comprised of a silica-based glass host having semiconductormicrocrystallites embedded within, said frequency doubler means havingan internally generated periodic quasi-phase matched electric field thatfrequency doubles the output of said laser diode means.
 15. An opticaldevice for frequency doubling optical radiation, said device including aglass substrate having semiconductor microcrystallites contained within,the glass substrate having a waveguide structure formed within at leastone surface thereof for guiding radiation having a first wavelength andfor converting a portion of the guided radiation to a second wavelength,wherein said glass substrate includes an internally generated periodicquasi-phase matched electric field that interacts with the guidedradiation for converting the portion of the guided radiation to thesecond wavelength.